Processes for the production of nitrobenzene by adiabatic nitration of benzene by means of nitric acid in the presence of sulfuric acid have already been known for some time.
Such a process was filed in its basic form, for example, as early as 1939 and granted as a patent in 1941 as U.S. Pat. No. 2,256,999.
Current, basic forms are described, for example, in U.S. Pat. No. 4,091,042, U.S. Pat. No. 5,313,009 and U.S. Pat. No. 5,763,697.
A common feature of the substantially adiabatic processes described therein is that the actual starting materials of the nitration—benzene and nitric acid—are reacted in the presence of large excesses of sulfuric acid.
Furthermore, in all the above-mentioned processes of the prior art, the nitric acid is premixed with the excess of sulfuric acid. A mixture so obtained, comprising substantially sulfuric acid and nitric acid, is referred to therein, as well as within the context of the present invention, as mixed acid.
The reaction of benzene with nitric acid to give nitrobenzene and water is highly exothermic (ΔH=−117 kJ/mol), and in the processes mentioned above the large excess of sulfuric acid takes up almost quantitatively the reaction enthalpy in the form of heat liberated during the reaction, as well the water formed thereby.
It is likewise a common feature of the above-mentioned processes that at least part of the heat from the reaction enthalpy stored in the reaction product is used to re-concentrate the sulfuric acid, which becomes highly diluted with water in the course of the reaction, by evaporation of the water.
Accordingly, in U.S. Pat. No. 4,091,042, the process is carried out in four series-connected stirred vessels, the reaction product leaving the fourth stirred vessel being passed into a phase separator, which is likewise operated continuously, in which the so-called spent acid, which is substantially aqueous, is separated from the so-called crude nitrobenzene, which is substantially organic.
In connection with the present invention, spent acid denotes a substantially polar, aqueous mixture comprising water and sulfuric acid, in which portions of nitrobenzene may still be dissolved in the sulfuric acid, and crude nitrobenzene denotes a substantially non-polar, organic mixture comprising benzene and nitrobenzene, in which portions of sulfuric acid and water may still be dissolved in the nitrobenzene.
In U.S. Pat. No. 4,091,042, the spent acid is concentrated in a flash evaporator using the heat of reaction stored in the spent acid. According to U.S. Pat. No. 4,091,042, this concentration takes place at 90° C. and about 80 mbar.
According to U.S. Pat. No. 4,091,042, the crude nitrobenzene is also worked up further by being passed continuously into a four-stage counter-current extraction washer, in which acidic constituents such as sulfuric acid residues and secondary reaction products, such as, for example, dinitrophenol and picric acid, are extracted by contact with a sodium carbonate solution.
The crude nitrobenzene thus purified is finally subjected to steam distillation, in which the remaining benzene is separated off in order to be returned to the nitration.
U.S. Pat. No. 4,091,042 accordingly does not disclose a multi-stage evaporation.
In the process described in U.S. Pat. No. 5,313,009 for the adiabatic production of nitrobenzene, nitric acid is also mixed with a large amount of sulfuric acid to form the mixed acid, and benzene is metered into the mixed acid, the benzene then reacting with the nitric acid to give water and substantially nitrobenzene.
According to the process of U.S. Pat. No. 5,313,009, the temperature of the reaction, the concentration of benzene, the concentration of nitric acid and the concentration of sulfuric acid are so chosen that—in contrast to the process according to the above-mentioned U.S. Pat. No. 4,091,042—a substantially nitric-acid-free mixture of benzene, nitrobenzene, sulfuric acid and water is already obtained after a first reaction zone. That is to say, according to U.S. Pat. No. 5,313,009, the process is so operated that the conversion of nitric acid is quantitative. To that end, benzene, is used in at least stoichiometric amounts, based on the amount of citric acid.
The substantially nitric-acid-free reaction mixture obtained downstream of the reaction zone is also fed in U.S. Pat. No. 5,313,009 to a single phase separator, in which the crude nitrobenzene is separated from the spent acid.
The spent acid is fed, analogously to the process of U.S. Pat. No. 4,091,042, to a flash evaporator, in which the water is evaporated from the spent acid likewise at reduced pressure compared with ambient pressure and at elevated temperature. Here too, at least part of the heat from the reaction products is used for the concentration of the sulfuric acid in said flash evaporator.
U.S. Pat. No. 5,313,009 merely describes that the concentrated acid obtained from the flash evaporation approximately identical in terms of its composition to that used at the beginning, of the process. U.S. Pat. No. 5,313,009 does not disclose the exact pressure of the flash evaporation or the exact temperature of the flash evaporation.
Accordingly, U.S. Pat. No. 5,313,009 likewise does not disclose a multi-stage flash evaporation of the spent acid.
In the process according to U.S. Pat. No. 5,763,697 too, mention is made only of a “purification and concentration stage”, but it is not disclosed in U.S. Pat. No. 5,763,697 precisely how that purification and concentration is carried out. For the rest, the process of U.S. Pat. No. 5,763,697 is similar to that of the above-described U.S. Pat. No. 5,313,009.
The above, generally known processes of the prior art are, therefore, all characterised in that the further treatment of the spent acid takes place in a single stage in a flash evaporation under reduced pressure, at least part of the heat of which comes from the adiabatic reaction of benzene with the nitric acid.
As a result, in addition to the desired evaporation of the water in order to concentrate the sulfuric acid, further constituents of the spent acid are generally evaporated at the same time, because the desired rate of separation of the water must be achieved in one stage, which, with the available heat provided by the reaction enthalpy, can be achieved only by the pronounced and in particular rapid pressure reduction.
Accordingly, in addition to the evaporation of water, there also occurs, for example, the outgassing of nitric oxides, which form as products of secondary reactions, such as, for example, oxidations, during the nitration reaction. Nitrogen monoxide and dinitrogen monoxide have boiling points of about −150° C. and −90° C., respectively, so that they cannot be recovered in the further process by condensation.
In addition to these there also form, as a result of the single flash evaporation, further products, referred to in the following as non-condensable gases, which, on account of their low boiling point, cannot be recovered economically. These result in disadvantages in particular in relation to the dimensioning of the condensation devices and devices for generating the required low pressure. In both cases, because of the significantly increased volume flows, it is necessary to use devices with larger dimensions, which, if nothing else, adversely affect the profitability of such processes.
DE 19636191 addresses the above-mentioned problems with a view to particular individual problems relating to environmental pollution. Unlike the above-mentioned prior art, it describes a multi-stage purification process for the spent acid. The multi-stage process according to DE 19636191 comprises a first step in which so-called steam-volatile organic compounds are removed by the passage of steam (“steam stripping”) countercurrently at a pressure in the range of from 200 mbar to 1000 mbar, following which, in a second step, at the same pressure, a first evaporation of the water from the remaining sulfuric acid is carried out and, in a third step, the evaporation of the water is continued repeatedly at further reduced pressure.
In contrast to the processes from the preceding prior art, the heat for the evaporation of the water is supplied from outside and from a different source. The use of the heat from the nitration for the evaporation of the water from the spent acid is not disclosed.
EP 0415951 also describes a multi-stage evaporation of the water from the spent acid, which, analogously to DE 19636191, is preceded by an additional purification step in the form of steam stripping. Like DE 19636191, EP 0615951 does not disclose the use of the heat from the adiabatic nitration.
The approach described in EP 0615951 and DE 19636191 is also to be found summarised in Ullmann (“Sulfuric acid and sulfur trioxide”, 2005, Wiley VCH, p. 51 to 52). Here, it is described that a multi-stage concentration of sulfuric acid according to Lurgi, (FIG. 42), in which the pressure falls from 1 bar to 70 mbar in order to produce a temperature gradient, allows an energy saving to be made. In the variants presented therein, the heat of condensation from the recondensation of the vapour of one evaporation stage is in each case used to heat the preceding stage.
However, a multi-stage concentration of sulfuric acid by evaporation as described in DE 19636191, EP 0615951 and Ullmann (2005) is expedient from the point of view of energy only if the concentration difference of the spent acid between the various stages of the concentration is sufficiently great, so that the heat of condensation from the recondensation of the vapour of the preceding evaporation stage (“vapours”) can be used to heat the following stage.
In the case of the concentration of the spent acid from the nitration of benzene, that is generally not the case.
The technical teachings of DE 19636191 and EP 0615951 relate in particular to the treatment of spent acids from the nitration of toluenes, which are then to be separated from the spent acids. Unlike the nitrated benzenes of the present invention, such mono- and/or di-nitrotoluenes do not have suitable saturation temperatures, or suitable concentration differences at the saturation temperatures, so that it is not possible to apply the technical teachings contained therein to the problem of the production of nitrobenzene and the problem contained therein of separating and concentrating the sulfuric acid containing those impurities.
Accordingly, the object continues to be to provide a process which permits the production of nitrobenzene by nitration of benzene with nitric acid in the presence of sulfuric acid and which does not have the mentioned disadvantages of the prior art in relation to the production of nitrobenzene.
In particular, the process is not to have the disadvantages associated with the formation of considerable amounts of non-condensable gases dissolved in the acid—the substantially increased energy outlays for the dissipation thereof in the case of single-stage treatment—and therefore allow the devices for generating low pressure to be of smaller dimensions and accordingly less expensive.
A further problem which occurs in the prior art and has not hitherto been solved is that, owing to the very low pressures in the flash evaporators of the prior art, the saturation temperature of the vapour stream produced (vapours) is frequently close to the supply temperature of the cooling medium of conventional cooling systems (water), so that large heat exchanger surfaces are required for the condensation of that stream. Because the process pressure is close to the saturation temperature of the water, a considerable portion of the gas stream consists of steam, as a result of which the capacity requirements of the vacuum pump increase still further.
Overall, therefore, the problem according to the prior art is to provide a process for the production of nitrobenzene by nitration of benzene with nitric acid in the presence of sulfuric acid which allows the investment costs of the plant for carrying out the process to be reduced significantly.